A superabsorbent material in general refers to a water-swellable, water-insoluble, material capable of absorbing at least about 10 times its weight, and up to about 30 times or more its weight in an aqueous solution containing 0.9 weight percent sodium chloride solution in water. The present invention relates to superabsorbent polymer particles, which absorb water, aqueous liquids, and blood, and a method to make the superabsorbent polymer and particles. The acronym SAP may be used herein, and is generally used in the industry, in place of superabsorbent polymer, superabsorbent polymer composition, superabsorbent polymer particles, or variations thereof.
A superabsorbent polymer is a cross linked partially neutralized polymer that is capable of absorbing large amounts of aqueous liquids and body fluids, such as urine or blood, with swelling and the formation of hydrogels, and of retaining them under a certain pressure in accordance with the general definition of superabsorbent material. A superabsorbent polymer composition is a superabsorbent polymer that has been surface treated that may include surface cross linking, surface treatment, and other treatment. Commercially available superabsorbent polymer compositions include cross linked polyacrylic acids or cross linked starch-acrylic acid graft polymers, in which some of the carboxyl groups are neutralized with sodium hydroxide solution or potassium hydroxide solution. Superabsorbent polymer composition particles are particles of superabsorbent polymers or superabsorbent polymer compositions, and generally have a particle size of from about 150 microns to about 850 microns. A comprehensive survey of superabsorbent polymers, and their use and manufacture, is given in F. L. Buchholz and A. T. Graham (editors) in “Modern Superabsorbent Polymer Technology,” Wiley-VCH, New York, 1998.
A primary use of SAP and SAP particles is in sanitary articles, such as babies' diapers, incontinence products, or sanitary towels. For fit, comfort, and aesthetic reasons, and from environmental aspects, there is an increasing trend to make sanitary articles smaller and thinner. This is being accomplished by reducing the content of the high volume fluff fiber in these articles. To ensure a constant total retention capacity of body fluids in the sanitary articles, more SAP content is being used in these sanitary articles.
The present invention relates generally to superabsorbent polymer composition particles having a particle size of about 150 microns or less, which are generally known to those skilled in the art and referred herein as superabsorbent polymer fines, SAP fines, or fines. The present invention relates to recycling the superabsorbent polymer fines during the production process. More particularly, the invention relates to reprocessing superabsorbent polymer fines generated in the production of superabsorbent polymer particles.
In particular, SAP fines are generally generated during the cutting, commination (pulverization), and grinding steps of the dried polymer gel. Superabsorbent polymer fines are considered to be undesirable in sanitary articles including infant diapers and adult incontinence devices. Such SAP fines may migrate in the device before use and exhibit gel blocking in application. In addition, it has been found that superabsorbent polymer particles made including a portion of recycled superabsorbent polymer fines have properties including absorption as measured by the Centrifuge Retention Capacity test (CRC), absorbency under load (AUL), and permeability (GBP) that are less desirous than the superabsorbent polymer particles that are made from a process that does not include recycling superabsorbent polymer fines.
U.S. Pat. No. 5,514,754 is directed to a process for increasing the size of fine particles of SAP (cross-linked polyacrylic acid, cross-linked polyacrylate, and/or cross-linked partially neutralized polyacrylic acid) and, more particularly, to a process of impregnating the SAP particles with an aqueous monomer solution of acrylic acid and/or metal salts of acrylic acid. Impregnation is achieved by intimately mixing the SAP fines (e.g., having a particle size less than about 300 μm) with the monomer solution to form a paste having a viscosity of, e.g., about 20,000 to about 5,000,000 centipoise. Polymerization of a monomer within, and drying of, the paste then is achieved, e.g., by heat.
It is an object of the present invention to provide superabsorbent polymer particle compositions made by a process that includes recycled superabsorbent polymer fines wherein the superabsorbent polymer particle compositions have improved properties including an absorption capacity to retain fluids under no load, high absorption capacities to retain fluid under pressure as measured by absorption under load at 0.9 psi, and improved gel bed permeability.